1. Field of the Invention
This invention relates to a method and apparatus for forming an energy subtraction image. This invention particularly relates to a method and apparatus for forming an energy subtraction image, wherein an energy subtraction image having an altered image size is formed.
2. Description of the Related Art
Techniques for photoelectrically reading out a recorded radiation image in order to obtain an image signal, performing appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) . The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often performed on image signals detected from a plurality of radiation images of an object, which radiation images have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which components represent the image information recorded at corresponding sampling points in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
As one of the subtraction processing techniques described above, an energy subtraction processing technique has heretofore been known. In the energy subtraction processing technique, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy distributions. Specifically, by the utilization of the characteristics described above, a plurality of radiation images of an object, in which different images of a specific structure of the object are embedded, are formed with several kinds of radiation having different energy distributions. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure of the object. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037, and Japanese Unexamined Patent Publication No. 3(1991)-285475.
The energy subtraction processing technique may be classified into two kinds of techniques. One of the techniques is a two-shot energy subtraction processing technique, and the other is a one-shot energy subtraction processing technique.
With the two-shot energy subtraction processing technique, radiation having a certain energy distribution is irradiated to the object, and a radiation image is formed with the radiation carrying the image information of the object. Thereafter, radiation having a different energy distribution is irradiated to the object, and a radiation image is formed with the radiation carrying the image information of the object. The two radiation images having been formed with the two kinds of radiation having different energy distributions are then subjected to the subtraction processing. The two-shot energy subtraction processing technique has the advantages in that the difference in energy level between the two shots is capable of being kept large.
With the one-shot energy subtraction processing technique, two stimulable phosphor sheets are superposed one upon the other with an energy separating plate, or the like, which changes the energy distribution of radiation, intervening therebetween, and two radiation images are formed respectively with two kinds of radiation having different energy distributions on the two stimulable phosphor sheets and with a single, simultaneous exposure to radiation. The thus formed two radiation images are then subjected to the subtraction processing. The one-shot energy subtraction processing technique has the advantages in that a shift between the object images in the two radiation images is capable of being kept small. (The one-shot energy subtraction processing technique is described in, for example, U.S. Pat. No. 4,855,598.)
As one of apparatuses for performing the energy subtraction processing, a built-in type of radiation image recording and read-out apparatus has heretofore been proposed. The built-in type of the radiation image recording and read-out apparatus comprises a radiation image recording apparatus for recording radiation images of an object, a read-out apparatus for reading out the recorded radiation images, an image processing unit for performing the subtraction processing on the radiation images having been read out, and the like, which are built within a single apparatus. With the built-in type of the radiation image recording and read-out apparatus, for example, two stimulable phosphor sheets superposed one upon the other with an energy separating filter, which absorbs low energy components of radiation, intervening therebetween are located so as to stand facing the object. In this state, the stimulable phosphor sheets are exposed to the radiation carrying the image information of the object. In this manner, a radiation image (i.e., a low energy image) is formed with radiation, which contains a comparatively large quantity of the low energy components, on a first stimulable phosphor sheet, which is close to the object. Also, a radiation image (i.e., a high energy image) is formed with radiation, which has passed through the energy separating filter and contains a comparatively large quantity of the high energy components, on a second stimulable phosphor sheet, which is remote from the object. The thus formed radiation images are read out with a predetermined read-out density, and radiation image signals having thus been obtained are subjected to the subtraction processing.
In cases where an energy subtraction image is formed with the energy subtraction processing, operation processing has heretofore been performed by directly utilizing a plurality of radiation image signals, which represent the radiation images having been formed with several kinds of radiation having different energy distributions. Therefore, a pixel density of the formed energy subtraction image becomes equal to the pixel density of the original radiation images. However, in the stage of the subtraction processing, several frequency components are lost, and noise often occurs. Accordingly, the image quality of the formed energy subtraction image becomes worse than the image quality of the original radiation images, and the energy subtraction image need not necessarily have the pixel density identical with the pixel density of the original radiation images.
The image signal representing the energy subtraction image formed with the energy subtraction processing is transferred to an external device, stored in an external device, or utilized for outputting a visible image with an output device, or the like. In such cases, there is a strong demand for reducing the image signal storage capacity and keeping the processing time, which is required for the signal transfer or the outputting of the visible image, short.